The process of solidification of a liquid alloy is a very complex phenomenon involving a multitude of variables and scientific principles. The objective always is to produce, during the solidification processing, the specific metallurgical structure with definite morphological features that are conducive to or instrumental in developing, upon subsequent treatment, the desired properties.
The solidification process is basically a nucleation and growth process, requiring the considerations of several different parameters at the same time. Such parameters include the chemical free energy change between the solid and the liquid phases and its change with composition and temperature, the surface free energy of the solid-liquid interface, the elastic strain energy, the amount of superheating and undercooling, the latent heat of solidification, the microscopic and macroscopic heat flow and fluid flow considerations, the thermal conductivities of the phases and, above all, the interdependence of these parameters. The literature does not contain data for all cases and hence it is sometimes necessary to estimate the importance of a particular variable.
The growth stage that follows the nucleation stage is more complex. It is at this stage that the physical defects (such as the chemical inhomogeneities, the dislocations, the voids, the "undesirable" crystals, etc.) appear. At present, solidification processing is achieved through a variety of methods and techniques, from the simplest to the exotic In all cases the main objective is to develop a microstructure capable of producing the optimum properties desired. In superalloys, there are generally three types of microstructure: the equiaxed grains, the columnar grains, and the single crystal. It is important to note that experts in this field do not agree about the best structure for a particular property requirement.
The simplest structure to produce is the equiaxed grains which need to be smaller for better properties. It is difficult to control the size by the conventional solidification methods. Also, in some cases the grain boundary needs to be strengthened for better properties.
The columnar grains are developed along a crystallographic axis by directional solidification method. Such columnar-crystal structures reduce the grain boundary area and optimize the properties in anisotropic crystals. However, this form also has several disadvantages. First, the orientation of the grains diverge from the axial array such that the final orientation is quite different from the desired one. Second, the dendrite-spacing, porositites, size and number of precipitates are different at different distances form the chill-zone. Third, all nickel base superalloys contain aluminum and titanium which have an inherent tendency to float to the liquid zone due to their lower density (which is less than that of the alloy), thereby producing not only inhomogeneities and impurity segregation but also equiaxed grains from the broken dendrite-arms due to convection ahead of the solidification front. An attempt to control this so-called "freckle formation" with a critical temperature gradient has produced limited success. A high temperature gradient may cause additional problems in producing segregations and inhomogeneities.
The "single crystal" method produces a specific orientation over the entire crystal. It possesses some definite advantages in some cases. However, it is very expensive as is the directional solidification method.
The single crystal and uni-directional solidification methods produce a "single crystal" nickel based superalloy with low modulus material with the expected improvements in low cycle fatigue characteristics. The "single crystal" in superalloys contain, in general, several phases or at least two phases, the matrix and the intermetallic phases, consisting of the gamma and gamma-prime phases, respectively. In some cases, the "directionally solidified" conventional superalloy contains the two-phase matrix of gamma and gamma-prime together with the interdendritic material of gamma/gamma-prime eutectic, carbides and others. It is almost impossible to control the structural variants to an ideal situation.
Moreover, a truly single crystal should contain the phase(s), distributed over the entire crystal with a desired orientation. Such is not the case in superalloys produced with the present methods. The crystal orientation is generally quite random in the directions normal to the growth direction, producing widely different orientations in the transverse direction.
Above all, the micro and macro porosities are invariably present in the nickel based superalloys, irrespective of the solidification processing method used. Due to the lack of sufficient scientific knowledge, the basic factors involved in microporosity formation are not known. It may be related to the volume of the gamma-gamma prime eutectic or it may be related to the voids formed during the present solidification processing. The simple fact still remains that it needs to be controlled.
Also, in the case of a complex superalloy, there exists a good probability of higher diffusivity of certain elements to produce phases with undesirable characteristics, like lower melting point, lower strength, lower fatigue properties. This fact alone defeats the purpose of most solidification methods and results in a premature failure.
To summarize, the prior solidification methods of superalloys offer little or no control of the following important structural parameters regarding the gamma-prime phase:
1. The size of gamma-prime precipitate, which determines to a large extent the yield strength and creep strength.
2. The number of gamma-prime precipitates per unit area, which is determined by the number of nucleation centers, and which controls the size of the precipitates.
3. Its shape, which determines partly its inherent stability, and which depends on homogeneity as well as other factors.
4. The ordered nature of the gamma-prime, which offers resistance to dislocation motion.
Furthermore, the prior art methods do not suppress the gamma-prime phase and have it precipitate later, which would result in a large number of nucleation centers, allow control of the size and the number of gamma-prime precipitates, and make the average distance between the particles small. These factors, which will provide maximum strengthening of the alloy, are presently impossible to achieve by all current conventional solidification methods known.
It is not possible to control collectively all important parameters by any of the available methods. What is desired, therefore, is the development of a method in which control of all of the important variables in solidification processing is possible, and in which an alloy having an ideal or desired microstructure can be obtained that will have superior qualities in terms of strength and temperature resistance It is further desired that these desirable qualities are produced in a simple, efficient and inexpensive manner.